Process of etching and etching bath for nickel base alloys



United States Patent 3,232,8ti2 PROCESS 6F ETCHENG AND ETCHING BATH FUR NECKEL BASE ALLOYS fiainuel Young, in, La Mirada and Dominic C. Mitchoil, In, Arcadia, Calili, assignors to North American Aviation, Inc.

No Drawing. Filed Mar. 11, 1963, Ser. No. 264,042

8 @iaims. (Cl. 15618) The present invention relates to a novel and improved etching composition particularly useful in connection with nickel base alloys and a process of etching or chemical milling such metals.

Metallurgy has recently provided design engineers with many new nickel base alloys that are particularly useful in applications requiring high strength, hardness or toughness at elevated temperatures. Since these materials are often hard and tough and have a very high work hardening rate, they are difficult to machine by conventional techniques. In order to form complex shaped parts with good tolerance and high surface finish chemical milling of nickel base alloys is desired. Previously available etching processes and compositions when applied to many nickel base alloys have yielded parts of poor tolerance with a rough surface finish or containing extensive intergranular corrosion.

It is therefore an object of this invention to provide high quality etching processes and etchants which eliminate the deficiencies of the prior art compositions and processes.

The invention described herein comprises the novel compositions, processes, ingredients and improvements herein shown and described. In particular it involves an etching reagent comprising free hydrogen ion, free nitrate ion, dissolved chromium and iron and a wetting agent in an aqueous solution for the etching of nickel base alloys.

The present invention has for an object the provision of a novel and improved process for the etching or chemical milling of nickel base alloys.

A further object is the provision of a novel and improved etching composition which acts rapidly on nickel base alloys and which produces a desirable surface finish.

Still another object is the provision of a novel and improved etching composition and process for use with nickel base alloys which acts rapidly on the alloys and permits chemical milling to close tolerance Without substantial degradation of mechanical properties.

Other objects and advantages of the invention will be obvious from the matter set forth hereinafter or may be learned by practice of the invention, the same being realized by means of the compositions, processes and steps pointed out hereinafter.

There is provided according to the practice of this invention an etching composition which comprises an aqueous solution of nitric acid, hydrochloric acid, iron chloride, chromium chloride and a surface active wetting agent which is stable in the acid solution. In carrying out the principles of this invention there is also provided an etching process which may be used with suitable corrosion resistant materials for the chemical milling or etching of nickel base alloys. The etching compositions and processes are also useful in removing surface layers or thin scale often present on the surface of these alloys.

Referring now in detail to the composition of material which forms a part of the present invention the etching bath comprises an aqueous solution of nitric acid, hydrochloric acid, ferric chloride, chro-mic chloride, and a surface active wetting agent preferably an alkyl aryl sulfonate which is stable in the etching composition and is not rapidly decomposed by the strong acids present in the etching bath.

ice

An example of a preferred etching bath is prepared so as to have the following composition: free hydrogen ion, 3 Normal; free nitrate ion, 0.8 Normal; dissolved iron, 200 grams per liter; dissolved chromium, 25 grams per liter; an alkyl aryl sulfonate such as, for example, dodecyl benzene sulfonic acid or acid soluble salt thereof, 0.1 to 0.2 percent. It is also preferred that the etching composition 'be used at F. when etching parts mounted so that the surface being etched is in a vertical position and at about F. when the surface being etched is in a horizontal position. This etching bath is particularly preferred for the etching of mill-annealed or aged Rene 41.

The desired solutions are conveniently prepared by evaporating 45 Baum ferric chloride solution down to 60 'Baum, mixing 40 percent basic chromic chloride crystals and a wetting agent in the hot ferric chloride solution and slowly adding 10 Normal hydrochloric acid and 15 Normal nitric acid to bring the etchant to the required free hydrogen and nitrateion concentrations.

The concentrations of the essential ingredients may be varied within relatively narrow limits for the purposes of the present invention to provide a suitable etching composition for a variety of nickel base alloys. Acceptable operating ranges for the various components of the etching composition are as follows: free hydrogen ion, 2.4 to 3.5 Normal; free nitrate ion, 0.4 to 1.2 Normal; ferric ion, to 250 grams of iron per liter; chromic ion, 25 to 50 grams of chromium per liter; alkyl aryl sulfonate ion, 0.05 to 0.25 percent. The concentration of chloride ion introduced by the above materials in the aqueous solution is not critical and is a quantity suflicient to maintain stoichiometry of the bath. Although chloride ion is preferred it will be appreciated that it can be partially replaced with fluoride or other halide ion in the etching bath up to a point Where equal concentrations of fluoride and chloride ions are present in the etching bath. Higher concentrations of fluoride ion than 50 percent of the total halide ion produce rapid intergranular corrosion and severe pitting. Chloride ion is preferred in the bath since baths containing appreciable fluoride are subject to fuming and the etch rate is more difficult to control than it is with the chloride baths. Chloride ion alone is preferred additionally to avoid the difiiculties of chemical analysis when mixed halide ions are present in the bath.

The hydrogen ion concentration is preferably between 2.4 and 3.5 Normal since below 2.4 Normal areas of passivation occur on the surface of the metal being etched, yielding an uneven etching rate which gives lower tolerance and poorer surface finish then obtained in the preferred composition bath and baths containing hydrogen ion concentrations above 3.5 Normal are unstable at the higher concentrations of metal ions and some gassing of the bath is noticed indicating hydrogen evolution. Baths having nitrate ion concentrations below 0.4 Normal yield an etching rate too slow to be practical on many of the nickel base alloys such as, for example, Rene 41 which is in the mill-annealed condition, and nitrate ion concentrations above 1.2 Normal lead to passivation of localized areas in the alloys particularly those in the aged condition.

The concentration of dissolved iron should be in ex cess of 180 grams per liter in order to produce a smooth surface finish onthe etched material. No serious effect on the condition of the etched part or the performance of the etching composition is noticed when the iron concentration ranges upwards from 180 grams perliter, although it is unlikely to exceed about 250 grams per liter which is the approximate solubility limit of iron in the acid solution and it is preferred that the iron concentration be greater than 200 grams per liter.

.although not intolerable in the bath up to the solubility limit.

The concentration of wetting agent is preferably between 0.05 and 0.25 percent since if the concentration is below 0.05 percent the effect of the wetting agent is virtually undetectable and the etching composition produces channeling and passivation of the surface or" the alloy resulting in an uneven etching rate with consequent reduction in tolerance and surface finish. Increase of the Wetting agent concentration above 0.25 percent results in foaming of the etching bath. The preferred alkyl aryl sulfonate may be replaced by other acid stable wetting agents such as, for example, a fluorinated alkyl carboxylic acid, a sulfated fatty alcohol or a sodium alkyl sulfate of the types readily available commercially such as, for example, Orvus W.A. Paste or Petrowet WN.

The temperature of operation of the etching bath is preferably from about 120 to 150 F. since the rate of etching diminishes appreciably below about 120 F. and the rate becomes so rapid above about 150 F. that violent gas evolution occurs and etching may be uneven and the bath becomes dangerous to handle.

In case a slower acting etching bath is suitable for use.

excessive precipitation of the metallic salts occurs and great difficulty is encountered in redissolving the precipitated salts.

Other materials may be present in the etching composition to improve its characteristics or may be present without substantial detriment to the etching characteristics of the bath. Thus, for example, when nickel base alloys containing appreciable concentrations of tungsten or silicon are to be etched, a portion of the free hydrogen ion can be obtained by the use of hydrofluoric acid in place of a portion of the hydrochloric acid as has been described above, in which case the fluoride ion concentration contributes to the etching process. Since the nickel base alloys often contain appreciable amounts of alloying additions such as cobalt, molybdenum, titanium and the like, appreciable concentrations of these ions may accumulate in the etching composition as it is used in the chemical milling of these alloys. Dissolved material from the etched alloy is permitted to accumulate in the etching bath up to a concentration of about 80 grams per liter total dissolved metal without degradation of the properties of the etching bath. Quantities of dissolved metal in excess of about 80 grams per liter are detrimental to the surface finish of the etched parts.

A variety of nickel base alloys can be etched in the described etching compositions. Typical of these alloys are \en 41, Hastelloy R235, Hasteiloy X, Inconel 700, Udimet 700 and Waspalloy, the nominal compositions of which are set forth in Table I.

1 Max.

the nitrate ion concentration can' be reduced to trace amounts when the etcl ing is to be conducted on alloys more susceptible to passivation. Also, the hydrogen ion concentration may be reduced to as low as 1 Normal; however, further reduction in the hydrogen ion concentration produces a low etching rate and an extremely rough and pitted surface unacceptable for many applications. Likewise the concentration of the dissolved iron in the etching bath can be reduced to 100 grams per liter, however, further reduction in the iron concentration yields an exceptionally rough surface finish. The chromium concentration in the etching solution can be reduced to 10 grams per liter, however further reduction in the chromiurn concentration substantially reduces the tolerance of the etched parts.

Likewise the hydrogen ion concentration can be increased above the preferred range to 6 Normal, however, above 6 Normal hydrogen ion concentration the solution is unstable, particularly in the presence of large quantities of dissolved metal, and excessive generation of hydrogen is obtained. Likewise an etching composition can be employed in the practice of this invention wherein the nitrate ion concentration is as high as 2.5 Normal, however higher Normalities produce extensive passivation of the metal surface giving an uneven and generally unacceptable etched part.

The temperature of the bath can be as low as 100 P. which produces a low rate of etching which may be desirable for etching alloys susceptible to passivation or for etching very large pieces When the surface to be etched is mounted in a vertical position. Below about 100 F. the rate of etching by the bath is too low to be practical and In all of these alloys it may be convenient to etch the metal in a variety of heat treated conditions. This is particularly of consequence in the Rene 41 alloy since this is an age hardenable alloy the etching characteristics of which vary appreciably between the aged and the annealed conditions. Previously available etchants have not been universally applicable to the described materials, whereas the etching compositions set forth herein have been found to be suitable for both the aged and annealed conditions. The annealed condition is achieved in the Rene 41 alloy by heating the material at 1975 F. for onehalf hour followed by a water quench. In order to achieve the precipitation hardened condition the material is heated for one-half hour at 1975 F. and water quenched followed by aging for 16 hours at 1400" F. and air cooling. Conventional machining techniques can be used when the material is in the annealed condition but the rate of material removal is extremely low and if large areas are to be machined conventional procedures are impractical. In the aged hardened condition conventional techniques of machining are extremely ditficult and expensive, and grinding is one of the few ways available for removing material. Finished parts with good tolerance and high surface finish are readily prepared by the chemical milling processes carried out according to the practice of this invention.

In describing a chemical milling process the term tolerance is defined as the variation in thickness of material removed from a surface for a given total thickness of penetration of the ctchant. Thus, for example, a low tolerance etching procedure is defined as one having a maximum variation in penetration of plus or minus one mil per 80 mils total penetration, and higher tolerances are acceptable in some applications. A desirable etching reagent and process yields a smooth, regular surface not subject to pitting or intergranular corrosion. The quality of finish on a surface is described as the variation in surface irregularities spaced less than &9, inch apart, and the deviation from the mean surface averaged and expressed in units of micro-inches root mean square (R.M.S.).

The materials to be etched or chemical milled may be in the form of sheets, finished parts, forgings or castings and may contain areas previously machined. The surface of the part to be chemical milled can have a rougher surface finish than the finished part to be obtained after chemical milling since the etchant composition provided in the practice of this invention is one which does not cause pitting and a somewhat faster etching rate occurs on small size high spots than adjacent areas to level the slightly rough surface. In etching the materials a surface coating of impervious corrosion resistant material is applied to those areas of the part which are not to be etched. This impervious material is preferably one totally resistant to the corrosive action of the highly acid bath such as, for example, the commonly used lacquers, waxes, acid resisting elastomeric masking compositions or oriented polyethylene terephthalate films such as, for example, adhesive Mylar tape. These resists or maskings compositions are often applied to the entire surface and stripped from the areas to be etched or conversely are selectively applied to the surfaces onwhich etching is not desired.

In order to eliminate the effect of passivation on the surface of many of the nickel base alloys, in particular Ren 41, it has been found desirable to prepare or activate the surface of the alloy before immersion in the etching bath. The passivation of the surface apparently occurs due to exposure of the alloy surface to atmospheric oxida-.

tion and the production of a very thin oxide film on the surface. Preferably the surface is activated by pickling the part in 12 to 14 Normal hydrochloric acid prior to immersion in the etching bath and so immersed Without rinsing, drying or long exposure to air. Parts removed from the etching bath during the chemical milling process and exposed to air are preferably repickled in the concentrated hydrochloric acid in order to reduce and eliminate passivation upon reimmersion of the part in the etching bath. Concentrations of hydrochloric acid below 12 Normal are nearly ineffective in activating the metal surfaces.

The part to be etched is immersed in the etching solution operating in the temperature ranges recited above 6 time in the bath at the particular composition and temperature conditions in use. The etching rate can readily be determined empirically or can be noted from the etching rates set forth in the examples hereinafter recited. During the operation of the etching bath stirring so as to eliminate concentration and temperature gradients within the bath is desirable.

Numerous tests of etching compositions to establish the preferred bath parameters were conducted as exemplified in the examples of Tables II through V. Table 11 sets forth a series of etching tests performed on aged 4" x 4" x 84 mils thick panels of Ren 41 (annealed at 1975 F. one-half hour, water quenched, aged at 1400 F. 16 hours, air cooled) with one surface, the edges, and approximately one-fourth inch of the other surface adjacent the edges suitably masked with corrosion resistant material. All of the baths described in Table II had a concentration of dodecyl benzene sulfonate ion of 0.1%. The volume of etchant was three liters and unless otherwise stated in the table the bath was operated at 140 F.

The normality of hydrogen and nitrate ions in the bath was determined by conventional chemical analysis before and after the completion of the etching tests and is expressed herein as the average of these concentrations. The iron and chromium content was determined by the quantity of metal in the chloride salts used in preparing the solutions. The concentration of nickel in the bath is calculated from the weight of dissolved Ren 41 alloy based on 57% nickel in the Ren 41 alloy. The contribution of the other alloying ingredients in the Ren 41 parallels that of the nickel and has less effect on the operating parameters of the etching bath than does the nickel concentration. The etch rate is expressed in units of mils of material removed per surface exposed to the .bath per minute (M.S.M.). Also set forth in the table is the surface finish obtained after etching the panels to the final thickness selected for the particular panel. In the Remarks column it will be noted that several samples became passive after a period of exposure to the etching bath. Passiv'ation results in cessation of material removal from the surface and if localized results in an extremely rough surface. It should be noted in connection with this and subsequent tables that a surface finish of less than about 150 microinches R.M.S. indicates that little, if any, intergranular corrosion has occurred, which corrosion would yield not only a rough surface but also diminished mechanical properties.

TABLE II Etching of aged Relz Etchant Composition Etch Surface Final Panel Metal Content, Rate .Finish Thiek- Remarks N 0. 11* ion NO ion GmJLiter MSM Micro-In. ness, Normality Normality RMS Mils.

Fe Cr Ni 3. 96 1. 78 100. 25 0 0. 5 110 66 5. 00 0.86 100 25 0 0. 140 50 4. 0.83 100 25 8 0. 67 130 53 4. 10 0. 25 15 0. 44 72 2. 63 2. 94 100 25 0 0. 4 85 75 5.67 1. 32 50 25 0 0. 4 170 59 Temp. F. 4. 47 1. 10 50 25 6 0. 43 57 3. 75 1. 00 50 26 12 0. 66 125 50 2. 63 0.70 50 25 20 0. 33 63 Temp. 130 F. 2. 05 0. 54 50 25 25 0. 3 220 66 5. 43 1. 07 50 25 0 44 30 3. 55 0.48 50 25 12 34 170 42 2. 52 0. 14 50 25 22 31 220 55 6.05 2.17 50 25 0 1.2 110 55 Temp. 120 F. 4. 62 1. 70 50 25 0 1. 7 65 58 for a sufficient period of time to remove the desired amount of metal, the total thickness of metal removed The baths used in the examples set forth in Table II and the following tables were prepared by evaporating being a function of the etching rate obtained and total 75 45 Baum ferric chloride down to 60 Baum, dissolving a suitable amount of 40% chromic chloride crystals' in the hotferric chloride solution to obtain the proper proportions of iron and chromium in the result ingsolution, cooling the solution to about 150 F., adding an aqueous solution of dodecyl benzene sulfonic acid so as to yield the selected concentration of dodecyl benzene sulfonate ion in the completed solution, slowly adding both Normal hydrochloric acid and Normal nitric acid to bring the etchant to the required temperature, and free hydrogen and nitrate ion concentrations. The compositions so prepared were maintained at a somewhat elevated temperature until immersion of the panels to prevent precipitation of the excess metal salts. All panels described in the examples were activated in 12 to 14 Normal hydrochloric acid before immersion in the etching solutions.

ably masked on and near the edges. The composition of the bath and other measured quantities Were determined and defined as described previously. The concentration of dodecyl benzene sulfonate ion in the etching bath employed for the examples set forth in Table IV was 0.2 percent. Additional designations L and T are used in indicating the surface finish obtained in the etched material. L refers to the longitudinal direction of the test material, that is, parallel to the rolling direction of the original sheet, and T represents transverse, that is, transverse to the rolling direction in the original sheet. The baths used to obtain the data in Table IV were prepared in the same manner as previously described and as in the examples set forth in the other tables the panels Were all mounted with the suriace to be etched in a horizontal position facing the top surface of the bath.

TABLE IV Etchzng of aged Ren 41 Etchant Composition Surface Finish Etch Micro-In. RMS Panel N0. Metal Rate Final 11+ ion N 03- ion Content, GmJLiter MSM Thickness, Normality Normality Mils Fe Cr Ni L T *Panel tipped over, passive areas.

Table III sets forth additional examples of etching of aged Ren 41 alloy similar in all respects to the examples set forth in. Table 11 except that the concentration of dodecyl benzene. sulfonate ion in the etching bath was 0.2 percent. Tensile tests were conducted on samples prepared from the panels etched in the described tests and in no case did the tensile strentgh nor the elongation diminish by more than 5 percent.

Table V sets forth the etching of additional panels of annealed Rene 41 of substantially larger size than the examples set forth in the preceding tables. Panels 1, 2 and 3 were chemical milled with the surface being etched in a vertical position. Samples A, B and C had the surface being etched in a horizontal position and facing the top surface of the bath. In all of the tests set forth in Table V the concentration of dissolved iron in the bath TABLE III Etching of aged Rem, 41

Etchant Composition Etch Surface Final Panel Metal Content, Rate Finish Thick, Remarks No. H+ ion NO ion Gm./Litcr MSM Micro-In. ness, Normality Normality RMS Mils.

' Fe Cr Ni 12-R-1 2. 15 0.51 200 50 0 0. 7 45 58 12-R2 1. 72 0.26 200 50 6 0.6 33 47 F. Temp.

Run. 12R3 1. 53 0. 10 200 50 14 0.2 33 71 12R4. 1. 31 0. 08 200 50 17 0.12 50 71 13-1 -c 4. 24 1. 82 200 50 0 0. 17 45 73 13 R.1 4.19 1. 50 200 50 3 0. 9 60 59 14-1 3.63 l. 11 200 50 0 0. 6 38 54 Table IV sets forth several examples of etching tests conducted on annealed samples of Rene 41 (annealed at 1975 F. for one-half hour, Water quenched). This series was 200 grams per liter, the concentration of dissolved chromium was 25 grams per liter and the concentration of dodecyl benzene sulfonate ion Was 0.2 percent. Other of tests was conducted on panels 3 X 4 x 92 mils suit- 75 parameters of the etching bath and the results obtained are the same as set forth in the other tables. The column entitled Induced Tolerance indicates the variation in thickness of the panel that was due solely to the chemical milling process and not due to the original tolerance 10 4. A process for forming a metal structure from a nickel base alloy comprising:

masking a portion of the alloy with. an acid resistive material,

variation in the panels before etching. subjecting the uncovered portion of the alloy to the ac- TABLE V Etching of aged Ren 41 Etchant Composition Surface Finish Panel Thickness, Mils Etch Micro-In. RMS Induced Panel No. Panel Size Etchant Rate Tolen,

Inch Temp, F MSM Mils H+ lOIl NOr ion L T Initial Final Normality Normality 12x12 115 2. 0. 84 93. 0-94. 0 40. 5-43. 0 :l:. 8 12x12 115 1. 87 0. 72 14 50 93. 0-94. 0 41. 0-42. 5 :b. 3 12x12 115 1. 70 0. 53 .13 30 35 93. 0-94. 0 40. 0-43. 0 5:1. 0 32x36 140 2. 48 0. 77 4 90. 8-96. 0 17. 7-21. 2 i1. 2 4 45 50 90. 8-96. 0 42. 5-45. 5 :i:. 9 32x36 140 2. 19 0. 64 3 85 100 91. 0-92. 5 21. 2-25. 6 i1. 5 32x36 115 2. 05 0. 43 1 75 150 90. 4-93. 0 18. 7-28. 3 i3. 5

The bath compositions and parameters described in the tables were investigated relative to Ren 41 in the aged and annealed conditions since the etching characteristics of these materials generally spans the etching characteristics of the other nickel base alloys useful in heat resisting applications. Etching of nickel base alloys such as .Hastelloy R235, Hastelloy X, Inconel 700, Udimet 500,

Udimet 700 and Waspalloy in baths of the above described compositions yield etch rates, surface finishes, and induced tolerances essentially equal to those enumerated in more detail in the tables.

It is to be understood that the above described examples are merely illustrative of the application of the principles of this invention. Those skilled in the art may readily devise other variations that will embody the principles of the invention. It is therefore to be understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: 1. A process for etching nickel base alloys which comprises:

subjecting the alloy to the action of a bath comprising: hydrogen ion in a concentration from 1 Normal to 3.5 Normal, nitrate ion in a concentration less than .5 Normal, dissolved ferric ion in a concentration greater than 100 grams per liter, dissolved chromium as chromic ion in a concentration greater than 25 grams per liter, a total concentration of other dissolved metals of less than 80 grams per liter, sufficient halide ion to maintain bath stoichiornetry, and an acid stable Wetting agent in a concentration from 0.05 percent to 0.25 percent.

2. An aqueous etchant for nickel base alloys comprising:

hydrogen ion in a concentration from 2.4 Normal to 3.5 Normal,

nitrate ion in a concentration from 0.4 Normal to 1.2

Normal,

dissolved iron in a concentration from 180 to 250 grams per liter,

dissolved chromium as chromic ion in a concentration greater than 25 grams per liter, and

an alkyl aryl sulfonate ion in a concentration from 0.1

percent to 0.2 percent.

3. A process for etching nickel base alloys comprising:

subjecting the alloy to the action of an aqueous solution Which contains hydrogen ion in a concentration from 2.4 Normal to 3.5 Normal, nitrate ion in a concentration from 0.4 Normal to 1.2 Normal, dissolved iron in a concentration from 180 to 250 grams per liter, dissolved chromium as chromic ion in a concentration greater than 25 grams per liter, and an acid stable Wetting agent in a concentration from 0.1 percent to 0.2 percent.

tion of an aqueous solution of 12 Normal to 14 Normal hydrochloric acid,

subjecting the uncovered portion of the alloy to the action of an aqueous solution which contains hydrogen ion in a concentration from 2.4 Normal to 3.5 Normal, nitrate ion in a concentration from 0.4 Normal to 1.2 Normal, dissolved iron in a concentration from 180 to 250 grams per liter, dissolved chromium as chromic ion in a concentration greater than 25 grams per liter, and an acid stable Wetting agent in a concentration from 0.1 percent to 0.2 percent.

5. An aqueous etchant for nickel base alloys consisting essentially of hydrogen ion in a concentration of about 3 Normal,

nitrate ion in a concentration of about 08 Normal,

dissolved iron in a concentration of greater than 200 grams per liter,

dissolved chromium as chromic ion in a concentration of greater than 25 grams per liter,

a total concentration of other dissolved metals of less than grams per liter,

suflicient chloride ion to maintain bath stoichiometry,

and

a dodecyl benzene sulfonate ion concentration from 0.1

percent to 0.2 percent.

6. A process for etching nickel base alloys comprising:

subjecting the alloy to the action of a heated aqueous solution comprising hydrogen ion in a concentration of about 3 Normal, nitrate ion in a concentration of about 0.8 Normal, ferric ion in a concentration of greater than 200 grams dissolved iron per liter, chromic ion in a concentration of greater than 25 grams dissolved chromium per liter, a total concentration of other dissolved metals of less than 80 grams per liter, suflicient halide ion to maintain bath stoichiometry, and an acid stable Wetting agent in a concentration from 01 percent to 0.2 percent.

7. An aqueous etchant for nickel base alloys comprising:

hydrogen ion in a concentration from 2.4 Normal to 3.5 Normal, nitrate ion in a concentration from 0.4 Normal to 1.2 Normal, dissolved iron in a concentration greater than grains per liter, dissolved chromium as chromic ion in a concentration greater than 25 grams per liter, a total concentration of other dissolved metals of less than 80 grams per liter, sutfi cient halide ion to maintain bath stoichiometry and an acid stable wetting agent in a concentration from 0.05 percent to 0.25 percent.

8. An aqueous etchant for nickel base alloys consisting essentially of hydrogen ion in a concentration from 2.4 Normal to 3.5 Normal, nitrate ion in a concentration from 0.4 Normal to 1.2 Normal, a dissolved iron concentration of greater than 200 grams per liter, a dissolved 1 1 1 2 chromic ion concentration of greater than 25 grams 2,955,027 10/1960 Newell 252-793 X per liter, a total concentration of other dissolved 3,057,765 10/ 1962 La Bod et a1 156 18 metals of less than 80 grams er liter, sufficient halide 3,0 0,07 0 2 Kinder 15 .43 'iontovmaintain bath stoichiometry and an acid stable wetting agent concentratinn from 0.65 percent to 5 F REI N TS 0125 Percent 513,130 5/1955 Canada.

References Cited by the Examiner 649510 10/1962 Canada UNITED STATES PATE T ALEXANDER WYMAN, Primary Examiner.

2613165 10/1952 Fischer. 1O 1 9 a B E E E 1: 2,904,413 9/1959 'Hampe1 156-20 XR JACO ST 

1. A PROCESS FOR ETCHING NICKEL BASE ALLOYS WHICH COMPRISES: SUBJECTING THE ALLOY TO THE ACTION OF A BATH COMPRISING: HYDROGEN ION IN A CONCENTRATION FROM 1 NORMAL TO 3.5 NORMAL, NITRATE ION IN A CONCENTRATION LESS THAN .5 NORMAL, DISSOLVED FERRIC ION IN A CONCENTRATION GREATER THAN 100 GRAMS PER LITER, DISSOLVED CHROMIUM AS CHROMIC ION IN A CONCENTRATION GREATER THAN 25 GRAMS PER LITER, A TOTAL CONCENTRATION OF OTHER DISSOLVED METALS OF LESS THAN 80 GRAMS PER LITER, SUFFICIENT HALIDE ION TO MAINTAIN BATH STOICHIOMETRY, AND AN ACID STABLE WETTING AGENT IN A CONCENTRATION FROM 0.05 PERCENT TO 0.25 PERCENT. 